FIG. 1 illustrates a block diagram of a parallel optical transceiver module 2 currently used in optical communications, which has multiple transmit and multiple receive channels. The transceiver module 2 includes a transmitter portion 3 a receiver portion 4. The transmitter and receiver portions 3 and 4 are controlled by a transceiver controller 6. The transmitter portion 3 comprises components for transmitting data in the form of amplitude modulated optical signals over multiple fibers (not shown). The transmitter portion includes a laser driver 11 and a plurality of laser diodes 12. The laser driver 11 outputs electrical signals to the laser diodes 12 to modulate them. When the laser diodes 12 are modulated, they output optical signals that have power levels corresponding to logic 1s and logic 0s. An optics system (not shown) of the transceiver module 2 focuses the optical signals produced by the laser diodes 12 into the ends of respective transmit optical fibers (not shown) held within a connector (not shown) that mates with the transceiver module.
Typically, a plurality of monitor photodiodes 14 monitor the output power levels of the respective laser diodes 12 and produce respective electrical feedback signals that are fed back to the transceiver controller 6, which processes them to obtain respective average output power levels for the respective laser diodes 12. The controller 6 outputs controls signals to the laser driver 11 that cause it to adjust the bias current signals output to the respective laser diodes 12 such that the average output power levels of the laser diodes are maintained at relatively constant levels.
The receiver portion 4 includes a plurality of receive photodiodes 21 that receive incoming optical signals output from the ends of respective receive optical fibers (not shown) held in the connector. The optics system (not shown) of the transceiver module 2 focuses the light output from the ends of the receive optical fibers onto the respective receive photodiodes 21. The receive photodiodes 21 convert the incoming optical signals into electrical analog signals. The transceiver controller 6 and/or other circuitry (not shown) of the transceiver module 2 processes the electrical signals to recover the data represented by the signals.
The laser driver 11 is typically a separate integrated circuit (IC). The laser diodes 12 are typically also contained in a separate IC. The monitor photodiodes 14 are typically also contained in a separate IC. The transceiver controller 6 is also typically a separate IC. The receiver photodiodes 21 are also typically contained in a separate IC. In addition to these ICs, the transceiver module 2 typically also includes a receiver IC that processes electrical data signals corresponding to the electrical signals produced by the receiver photodiodes 21. These ICs and other components, such as resistors, capacitors and optical elements (e.g., lenses) are typically mounted on some type of circuit board, such as a printed circuit board (PCB) having a generally rigid substrate on which conductive traces are printed or a flexible circuit substrate on which conductive traces are printed or etched. Flexible circuit substrates used for this purpose are typically referred to as flex circuits. Some parallel optical transceiver modules use both a PCB on which some of the aforementioned components are mounted and a flex circuit on which others of the aforementioned components are mounted.
Typically, the PCB or flex circuit includes, or is otherwise in contact with, a heat spreader device, such as a layer of metallic material or a leadframe, for example. The purpose of the heat spreader device is to dissipate heat generated by the electrical components by spreading the heat out away from the components that can be adversely affected by heat. The heat spreader device is typically a generally planar device that spreads heat laterally in directions that are generally coplanar with the plane of the heat spreader device. The heat spreading function is particularly important with respect to the laser diode IC because the performance of a laser diode can easily degrade as its temperature increases. The laser diode driver IC, which is typically placed relatively close to the laser diode IC in order to avoid long lead lengths, generates a large amount of heat. One of the primary functions of the heat spreader device is to move the heat generated by the driver IC away from the laser diode IC so that the heat does not adversely affect the performance of the laser diodes contained in the laser diode IC.
In addition to heat dissipation considerations, there are other considerations that are typically taken into account when designing parallel optical transceiver modules, such as size and inductive coupling, for example. It is generally a design goal for the transceiver module to be small in size, or to have a small “footprint”. It is also typically a design goal to have relatively short lead lengths in order to prevent or minimize inductive coupling between adjacent electrical conductors (e.g., wire bonds), which can lead to noise and performance degradation. Placing the laser diode driver IC close to the laser diode IC helps to reduce the footprint of the module and, at the same time, to enable the lengths of the wire bonds that connect the pads of the ICs to be kept relatively short. Keeping these conductor lengths short helps to prevent or minimize inductive coupling between adjacent conductors. However, placing these ICs in close proximity to one another makes it more difficult to isolate the laser diode IC from the heat generated by the laser diode driver IC. Thus, there are tradeoffs that require some balancing between these and other design goals.
A need exists for a parallel optical transceiver module that has a relatively small footprint, is efficient in terms of space consumption, has good heat dissipation characteristics, and that can be manufactured at relatively low cost with relatively high yield.